Cirrus MD56xx Modem Commands

Cirrus, then Ambient and now Intel modems based upon the 56xx
chipset are V.90, x2, and V.34-compatible. The command documentation for 2 of
the important extended commands (+MS and +ES) are very hard to find. (Most
documentation available on the web is either outdated, or refers to a
hard-to-find Programmer's Reference Manual.) Here is a summary of the +MS and
+ES commands, along with related V.42/MNP commands.

+MS=p,a,mintx,maxtx,minrx,maxrxWherep = protocol: valid values include:
V34, X2, and V90. Also valid: V34S, V32B, V32, V23C, V22B, V22, V21, Bell212A,
and Bell103a = automode: 1 = only connect at rates/protocol specified; 0 = allow
fallback if connection cannot be achieved at specified ratesmintx: Minimum upstream rate defines the lowest data rate at which a
modem connection can occur. Setting <min rate> to 0 has one of two
meanings depending on the <automode> setting. When both <automode>
and <min rate> are set to 0, then the lowest data rate at which the
connection can occur is the lowest data rate specified by the <carrier>
parameter. If <automode> is set to 1 and <min rate> is set to
0, then the lowest data rate is 300 bps.maxtx: Maximum upstream rate defines the highest data rate at which a
modem connection can occur. If the <max rate> is set to 0, the modem
uses the DTE data rate or a slower <carrier> data rate as the highest
permitted connection data rate. This highest permitted data rate means the modem
attempts to connect at this data rate but can connect at a slower rate because
of line impairment. If <max rate> and <auto-mode> are set to 0
and the DTE data rate is below the lowest data rate supported by the modulation
rate, then the modems connection attempts always fail and the modem reports a
"NO CARRIER" message.minrx and maxrx: Same as above, except for downstream rate (Valid
only for V.34, x2, and V.90 connects).
Valid rates include: 0, 300, 1200, 2400, 4800, 7200, 9600, 12,200, 14,400,
16,800,19,200, 21,600, 24,000, 26,400, 28,800, 31,200, 33,333, 33,600, 37,333,
41,333, 42,666,44,000, 45,333, 46,666, 48,800, 49,333, 50,666, 52,000, 53,333,
54,666, 56,000, and 57,333 bps.

ATW3 will report the actual receive and transmit rates in
the CONNECT string. AT+MS? will report the current settings for
Modulation Select.

If the +MS=m parameters contain conflicting information like
"+MS=V34,1,14400,0" with a DTE data rate of 2400 bps, then the
modems connection attempts always fail and the modem reports a "NO
CARRIER" message. When the modem receives the +MS=m command, the
modem does not check for conflicts of valid parameter information.

+ES= Controls the Error
Correction protocol:

Buffer (Normal)
+ES=1, 0, 1

No error correction/data compression, but speed
buffering is supported.

1) MNP5 requires the modem to be configured for %C1.
2) V.42 bis requires the modem to be configured for "H3.
3) Refer to the \Cn and %An
commands for more information about Auto-reliable
mode.

The list of commands needed
to enter a specific error correction or data compression mode are as follows:

Protocol:

Command

V.42 bis with fallback to
MNP5,MNP24 or V.42:

+ES=3,0,2 "H3 %C1

V.42 bis with fallback to
V.42/MNP24:

+ES=3,2,4 %C0 "H3 -J1

V.42 bis only:

+ES=3,2,4 -J0 "H3

V.42 only:

+ES=3,2,4 -J0 "H0

MNP5 with fallback to MNP24:

+ES=4,4,6 %C1

MNP24 only:

+ES=4,4,6 %C0

V.42 and MNP Data Mode
Command Descriptions

%An (Default = 13)

Set Auto-Reliable Fallback Character: In
Auto-reliable mode (+ES=3,0,2) with auto-reliable fallback character
enabled (\C2), receipt of the fallback character from the line during the
V.42 detection phase causes the modem to switch to buffer (normal) mode. This
allows a remote user with a non-V.42 modem to immediately connect with a V.42
modem. A space or carriage return is usually chosen for the fallback character.

n = 0127 (ASCII character.)

%Cn (Default = 1)

MNP 5 Data Compression Control: This
command controls whether the data sent during the MNP frames is compressed using
the MNP Class 5 compression standard. MNP 5 data compression can improve
throughput by as much as 150%.

n = 0, 1
n = 0 No compression.
n = 1 MNP Class 5 compression.

\An (Default = 3)

MNP Block Size: This
command specifies the maximum number of data bytes in an MNP data frame. A
smaller frame size can improve throughput on high-impairment (noisy) telephone
lines.

Set Auto-Reliable Buffer: (requires
a license from Microcom) In Auto-reliable mode (+ES=3,0,2), this command
determines the fallback method and enables data buffering. The settings for this
command are used by the modem during the V.42 detection phase.

n = 02
n = 0* Does not buffer data.
n = 1 Reserved.
n = 2 Does not buffer data. Switches to Buffer (normal) mode upon receipt of
auto-reliable fallback character and passes it to the serial port. This feature
allows non-V.42 modems to immediately connect to a V.42 modem without data loss.

\Gn (Default = 0)

Set Modem Port Flow Control: In
Buffer (normal) mode (either +ES=1,0,1 or after fallback), this command
enables modem-to-modem flow control using XOFF (13h) to stop and XON (11h) to
start transmission between modems.